Copolyesters are widely used as extrusion and injection-molding resins for the fabrication of various articles for household or industrial use, including appliance parts, containers and auto parts. A majority of the copolyesters are composed of polyethylene terephthalate (PET) modified with other diols and/or dicarboxylic acid esters. Copolyesters are usually prepared according to copolyester forming conditions well known in the art. For example, a mixture of one or more aromatic dicarboxylic acids or ester forming derivatives thereof, and one or more diols is heated in the presence of esterification and/or transesterification catalysts at temperatures in the range of about 150.degree. C. to about 250.degree. C., in an inert atmosphere. During the esterification or transesterification reaction, the by-product water or alcohol is removed. Normally, the dicarboxylic acid or dialkyl derivative thereof is esterified or transesterified with the diol(s) at atmospheric pressure and at a temperature at the lower end of the specified range. Polycondensation then is effected by increasing the temperature and lowering the pressure while excess diol is removed from the mixture.
U.S. Pat. Nos. 5,124,435 and 5,095,089 disclose Poly(ethylene/1,4-cyclohexylenedimethylene terephthalate copolyesters (PETG) prepared by an esterification reaction or ester exchange reaction and a polycondensation reaction. Acetates of such metals as Zn, Mn, and Mg were employed as catalysts for the transesterification reaction of dimethylterephthalate (DMT), while oxides of such metals as Sb and Ge were used for the polycondensation of bis-2-hydroxyethyl terephthalate. Additionally, an alkali metal salt such as potassium carbonate or sodium acetate is incorporated into the catalyst formulation as a color stabilizer or masking agent to mask the yellow color generated during processing.
U.S. Pat. Nos. 2,578,660, 3,074,913, and 3,377,320 disclose polyesters prepared using an ester-interchange catalyst and a germanium compound as a polycondensation catalyst. The patents suggest that the use of germanium compounds as polycondensation catalysts produce polyesters which are free from color without deleteriously affecting the rate of polycondensation.
The process of the present invention has eliminated the transesterification of a dialkyl ester reaction and the esterification reaction in the production of copolyesters. Moreover, the copolyesters produced by the process of the present invention are clear and colorless, and thus, do not require the addition of color stabilizers or masking agents.